Voluntary Aadhaar authentication: consent-based use with alternate ID options; mandatory only when required by parliamentary law. An Aadhaar number is a non-reassignable random identifier; holders may voluntarily use it in physical or electronic form for authentication or offline verification with informed consent. Entities may be permitted to perform authentication only if they satisfy Authority-prescribed privacy and security standards and legal permission, and the Authority may require use of actual Aadhaar or a virtual identifier. Requesting entities must offer alternate identification methods and cannot deny service for refusal to authenticate, but mandatory authentication may occur if required by a law made by Parliament.